Use of Boron as a Tracer for Recycled Water in Brackish Aquifers

An investigation was conducted to develop a simple method for tracking the fate and transport of recycled water following recharge into a shallow brackish aquifer (caprock aquifer) in a coastal area on the Island of Oahu, Hawaii. Several naturally present chemical constituents including the boron isotopic signature ((delta) 11B) were used to characterize each of the caprock aquifer source waters and the recycled water. Because of the influence of seawater, only (delta) 11B could be used to clearly distinguish the recycled water from the source waters and the caprock aquifer water. Estimates of the mixing ratios of source waters in the aquifer were made and a method was developed to determine the fraction of wastewater present in a brackish water sample recovered from a monitoring well during recharge operations without addition of a tracer. This method can be adapted to monitor any other brackish aquifer subjected to wastewater recharge.